In an automated data storage library, numerous storage slots, or cells, arrayed within the library are used to hold data storage media, such as magnetic tape cartridges or cassettes, or optical disk cartridges. (The term "media" used herein refers to any portable housing structure containing any type of data storage media.) The storage slots are typically arranged in planar or cylindrical arrays of rows and columns. A picker, furnished with one or more grippers, is a robotic device which moves along a guideway in a horizontal motion, or about a pivot in a rotary motion, and moves vertically to access the various storage slots with the gripper, and transports selected data storage media amongst the storage slots and one or more read/write stations. Libraries also typically contain input/output stations or ports through which an operator may pass data storage media to be added to the storage array and through which the picker may pass data storage media to be removed from the data storage array. The operation of the picker is typically under the direct control of a library manager, which is a data processing controller typically situated in a library or in an external host controlling the library. The library controller is interconnected with one or more host computer systems, such as a mainframe or network computer, which issues commands to the library. The read/write stations may be interconnected with the host(s) and, after a data storage medium is delivered to the station, typically search for, and read selected data from or write data to the selected data storage medium under the control of a host.
Commands received by the library manager from the host(s) are often queued to permit the receipt of additional commands during the execution of the previous command so that the processing speed of the system is increased. Two grippers may be provided on a picker to further increase the speed of the system. An example of a library having queuing and a two gripper picker is illustrated in co-assigned U.S. Pat. No. 5,646,918, Dimitri et al., issued Jul. 8, 1997. Additionally, two pickers may be provided and may be operated simultaneously to increase the speed of the system. An example of a library having queuing and two pickers is illustrated in U.S. Pat. No. 5,513,156, Hanaoka et al., issued Apr. 30, 1996.
It is desirable that an automated data storage library operate at high efficiency, minimizing the picker motion. Thus, a two gripper picker may, as described in the '918 patent, conduct an "exchange" of media at a read/write station or at a storage slot, using one of the grippers to fetch a cartridge or cassette from a read/write station (or storage slot) and the other to deliver another cartridge or cassette to the read/write station (or storage slot). The "exchange" in the '918 patent comprises a combination of individual cartridge moves directed by commands received from the host(s). The combined commands involve, for example, a move of one cartridge from a storage slot to a read/write station and a move of another cartridge from the same read/write station to a different storage slot.
While an exchange of media may lead to greater efficiency in many cases where a single picker with dual grippers is employed, a library having two or more pickers may be unable to employ the technique. For example, the dual pickers in the '156 patent may be restricted in movement, so that it may not be possible for one picker to conduct an exchange where one of the cartridges is to be moved from or to an area where the other picker is located. Copending patent application Ser. No. 09/015272, Dimitri et al., filed Jan. 29, 1998, "Automated Data Storage Library Dual Picker Interference Avoidance," allows two pickers to conduct moves, including exchanges by one of the pickers, while avoiding interference between the pickers. In many instances, it is not efficient to attempt an exchange where two or more pickers are involved, even though interference may be avoided, for example, if one picker must be moved out of the way and idled so that another picker may conduct an exchange.